The influence of a mentoring program (Big Brothers-Big Sisters) on the peer relationships of foster youth in relative and nonrelative care was examined. Youth were randomly assigned to either the treatment or control condition, and changes in their peer relationships were assessed after 18 months. Foster parents were more likely than nonfoster parents to report that their child showed improved social skills, as well as greater comfort and trust interacting with others, as a result of the intervention. In addition, whereas the peer relationships of all nonfoster youth (N = 90) remained stable, treatment foster youth (N = 90) reported improvements in prosocial and self-esteem enhancing support, and control foster youth showed decrements over time. When the foster youth were differentiated further on the basis of their placement, a pattern of findings emerged in which treatment youth in relative foster care reported slight improvements in prosocial support, whereas treatment youth in nonrelative foster care reported slight declines. All foster youth in the control group reported decrements in peer support over time, with nonrelative foster youth reporting the sharpest declines. Implications for research and intervention are discussed.